1. Field of the Invention
The present invention relates to a method for making an optical fiber by drawing an optical fiber preform.
2. Description of the Background Art
An optical fiber can be made by a step of heating and softening a lower end side portion of a silica glass optical fiber preform (hereafter, also shortly called “preform”), and extending the softened portion by applying a tension so as to make a thin glass fiber and finally by a step of covering the circumference of the glass fiber with a resinous coating. Such step of making an optical fiber by reducing the diameter of a preform is called “drawing”, and the optical fiber thus drawn is rolled around a bobbin on the downstream of a path line by means of a take-up means such as a capstan roller.
In the past, it has been difficult to make an optical fiber having a completely concentric circular cross-sectional form of core and cladding, and it has been a problem to be solved. If a core or a cladding has a cross-section which is an elliptic or distorted circular form, the refractive-index profile will not have axial symmetry. This will cause differences in the group velocity between two polarized waves which perpendicularly intersect, travelling through an optical fiber, and consequently the polarization mode dispersion will become large.
Japanese Patent Application Publication No. H8-277139 describes a method of drawing an optical fiber preform into a fiber while rotating the preform. According to the method, the lower end side of a preform is heated, and the softened lower end portion of the preform is allowed to fall as a “dropping part” by means of its own weight. Thereafter, the glass extended by falling of the dropping part is set to a path line leading to a capstan roller, and the preform is rotated at a given rotational speed while the line speed is being raised to a regular line speed, and the drawing of an optical fiber is commenced. However, even when the drawing was conducted while the preform was rotated, there were cases where an optical fiber thus made was substandard such that a core or cladding had an elliptic or distorted circular cross-section, or the core was nonconcentric.